Theory of Operation
4-12 1705A Spectrum Monitor
U6 is an 8-bit D-typedata latch that drives all the front-panel LEDs and most of
the internalcontrol lines. Chip select forU6 is /WR (U2, pin 16) inverted (U7C)
and ANDed with Address14 (U2, pin 27) by U7A.
U10 is a dual 8-bit DAC (Digital-to-AnalogConverter) that generates the
horizontal andvertical readout signals. Theanalog current outputs of U10 are
pins 2 and 20, which areconverted to voltage by U12 A and B. Chip selectis
/WR (U2, pin 16) which is inverted(U7C) and ANDed with Address 7 in U7D.
Line A0 from U8, pin 19, is the DACA select (low enable). U5provides a
--5 volt analog voltage referencefor U10.
Tracerotation compensates for changes in the magnetic field surroundingthe
1705A. Q1and Q2 are emitter followers thatprovide the Trace Rotation current
to a coil around the crt, inside the shield. Thevoltage on the emitter of either Q1
or Q2 will develop acurrent through R19 to drive the coil. Currentamplitude
and polarity are controlled by the front-panel TR ACE ROTATIONscrewdriver
adjustment.
Q3 and Q100 provide a current sourcefor the graticule lights. Base voltage,
which controls theamount of current flowing in the light circuit, is set by the
front-panelSC ALE control. Jumper J100 on the Graticule Light board allows
the graticulelights to be disabled.
Front PanelDiagram 6The front-panelschematic shows all of the operational controls for the instru-
ment, including potentiometers,momentary push-button switches, and indicator
LEDs. All the push buttons are polled by the microprocessor,and the LEDs are
driven by output ports fromthe microprocessor. Also included on this diagram
is the +18 volt Block Down Converter (BDC) power supply.
The front-panel LED indicators are return ed to a current source (+5 V). When an
LED is lighted, there is acomplete circuit from the Light Driver (Diagram 5)
through the LED to the +5V supply. The five function switches (front-panel
push buttons) aresimple ground closures that are read by the microprocessor to
determinethe operating mode.
The eight front-panelcontrols determine voltages (in a range between --11.8V
and +11.8V) depending on circuit requirements. Each control works with a
specificcircuit on another diagram.